Hole drift-mobility measurements and multiple-trapping in microcrystalline silicon

We present photocarrier time-of-flight measurements of the hole drift-mobility in microcrystalline silicon samples with a high crystalline volume fraction; typical room-temperature values are about 1 cm²/Vs. Temperature-dependent measurements are consistent with the model of multiple-trapping in an exponential bandtail. While this model has often been applied to amorphous silicon, its success for predominantly crystalline samples is unexpected. The valence bandtail width is 31 meV, which is about 10-20 meV smaller than values reported for a-Si:H, and presumably reflects the greater order in the microcrystalline material. The hole band-mobility is about 1 cm²/Vs - essentially the same magnitude as has been reported for electrons and for holes in amorphous silicon, and suggesting that this magnitude is a basic characteristic of mobility-edges, at least in silicon-based materials. The attempt-frequency v is about 10⁹ s ^-1; this value is substantially smaller than the values 10 ¹¹ - 10¹² s^-1 typically reported for holes in amorphous silicon, but the physical significance of the parameter remains obscure.